1. Field of the Invention
The present invention relates to a method for the improved processing of metal sulfates. More particularly, it relates to the recovery of sulfur dioxide from the metal sulfate followed by reaction of the sulfur dioxide with a carbonaceous material to yield elemental sulfur and carbon monoxide. The carbon monoxide is recycled for use in the initial processing of the sulfate to yield sulfur dioxide.
2. Background
A wide variety of processes have been developed for the production of sulfur values from metal sulfates and sulfides. These processes serve to improve operating efficiencies and more recently, to reduce environmental problems caused by waste product (solid sulfate) accumulation. Various forms of sulfur are recovered from these processes including sulfur dioxide, hydrogen sulfide and elemental sulfur. Sulfur dioxide is the desired product when sulfuric acid production is the objective while safety and economic considerations make sulfur the product of choice when shipment is required.
In another area of concern, high-sulfur fuels are notorious for the formation of copious amounts of environmental contaminant. As a result, there has been underway for some time efforts to remove sulfur from these high-sulfur, liquid and solid fuels. Although metal sulfates have been used in the removal process, there has been, prior to this invention, no effort to combine the sulfur recovery from fuels with the solid sulfate accumulation problem in a process that provides maximum energy and sulfur value recovery.
One example of the environmental concern for solid waste sulfate accumulation is readily apparent in the fertilizer industry where gypsum (calcium sulfate dihydrate) is produced in large quantities as the by-product of the conversion of phosphate rock (apatite or calcium phosphate) to phosphoric acid using sulfuric acid. Mounds of such by-product gypsum pose severe environmental problems as acidified rainwater (often from the burning of sulfur containing fuels) produces large runoffs of soluble compounds from the accumulations of gypsum by-product. Because of these large stockpiles of waste gypsum, there is a continuing need for efficient gypsum processing to reduce these stockpiles in a manner that is energy and end product efficient.
Because of this need, a wide variety of processes has already been developed for the processing of metal sulfates such as gypsum (CaSO.sub.4). These processes focus on the recovery of lime (CaO) and a form of sulfur, typically either elemental sulfur (S) or sulfur dioxide (SO.sub.2) with the later often used in the production of sulfuric acid. These processes typically involve 1) the reduction of the metal sulfate to form the metal oxide and sulfur dioxide or 2) the reduction of the metal sulfate to the metal sulfide followed by an oxidation step for the conversion of the sulfide to sulfur and/or sulfur dioxide.
The reductive decomposition of calcium sulfate is more fully illustrated by the following equations: EQU CaSO.sub.4 +CO.fwdarw.CaO+CO.sub.2 +SO.sub.2 (1) EQU CaSO.sub.4 +H.sub.2 .fwdarw.CaO+H.sub.2 O+SO.sub.2 (2) EQU CaSO.sub.4 +4CO .fwdarw.CaS+4CO.sub.2 (3) EQU CaSO.sub.4 +4H.sub.2 .fwdarw.CaS+4H.sub.2 O (4)
Equations (1) and (2) are carried out under what is referred to as mildly reducing conditions, that is, with a relatively low amount of reducing agent such as carbon monoxide or hydrogen or both. Reactions (1) and (2) require heat, i.e., are endothermic, and are favored by higher reaction temperatures. Reactions (3) and (4) are carried out under strongly reducing conditions, that is, with a high amount of reducing agent such as carbon monoxide or hydrogen or both. Reactions (3) and (4) give off heat, i.e., are exothermic, and are favored by lower reaction temperatures. As noted by Wheelock (U.S. Pat. No. 3,087,790), the use of an excess amount of carbon dioxide with the reducing gas discourages the sulfide production of equation (3).
The reactions that take place in the oxidation of calcium sulfide include: EQU 2CaS+3O.sub.2 .fwdarw.2CaO+2SO.sub.2 (5) EQU CaS+2O.sub.2 .fwdarw.CaSO.sub.4 (6) EQU CaS+CaSO.sub.4 .fwdarw.2CaO+2SO.sub.2 (7) EQU 2CaSO.sub.4 .fwdarw.2CaO+2SO.sub.2 +O.sub.2 (8)
Reactions (5) and (6) are exothermic with reaction (5) being favored at higher temperatures. Reactions (7) and (8) are endothermic. Overall, equations (5)-(8) can be approximated by the following overall reaction: EQU CaS+O.sub.2 +CaSO.sub.4 .fwdarw.2CaO+2SO.sub.2 (9)
There have been a large number of efforts to maximize the efficiencies of sulfate reduction including an emphasis on the use of mild reduction (equations (1) and (2)) or strong reduction (equations (3) and (4)). The direction of these efforts is often determined by the use of the sulfur dioxide product and energy values involved.
If the source of the sulfate is from a process using sulfuric acid, e.g., wet process phosphoric acid production, it is often desirable to use the sulfur dioxide in a conventional sulfuric acid plant such as is done in Wheelock (U.S. Pat. No. 3,087,790), Campbell et al (U.S. Pat. No. 3,582,276), Foecking (U.S. Pat. No. 3,607,036), and Marten (U.S. Pat. No. 4,963,513). Although the on-site production of sulfuric acid from sulfate byproduct is convenient, the economic laws of supply and demand often produce an oversupply of sulfuric acid at one site and a scarcity at another. Since sulfuric acid contains only about one third sulfur and for safety reasons, it is often desirable to recover elemental sulfur from the sulfate. Also as noted in Kamlet (U.S. Pat. No. 2,863,726), it is more desirable to ship sulfur than sulfuric acid since often elemental sulfur or products derived from elemental sulfur, e.g., hydrogen sulfide, sodium sulfide, colloidal sulfur, sodium hydrosulfite, liquid sulfur dioxide, etc. are required at the final destination.
As a result, Gorin (U.S. Pat. No. 3,729,551) uses a two stage process in which CaSO.sub.4 is reduced with hydrocarbonaceous solids and air to afford CaS, hydrogen, and carbon monoxide. The CaS is oxidized with air to afford SO.sub.2 and the SO.sub.2 is reduced using hydrogen and carbon monoxide to afford sulfur using a Claus process. Although the Claus process is well known and used for the conversion of sulfur dioxide to sulfur, it also requires a large amount of capital equipment and substantial quantities of fuel.
Another set of reactions for the conversion of metal sulfates involves the initial formation of hydrogen sulfide which may then be converted to sulfur. Kamlet (U.S. Pat. No. 2,863,726), Orahood (U.S. Pat. No. 3,661,518) and Weston et al (U.S. Pat. No. 4,704,136) teach such a conversion. Kamlet forms sulfur and hydrogen sulfide in a method for Portland cement clinker production. Orahood teaches the conversion of CaSO.sub.4 to CaS which is converted to H.sub.2 S using CO.sub.2. Weston uses an eutectic of alkali and alkaline earth sulfates. The sulfates are reduced to sulfides and converted to H.sub.2 S by reaction with water and carbon dioxide. The H.sub.2 S is used to form sulfuric acid using a conventional contact type sulfuric acid plant or converted to sulfur using the Claus process.
Fuel production and the removal of sulfur impurities from liquid and solid fuels, have resulted in several processes involving the reaction of carbonaceous materials and sulfur dioxide. In Kertamus et al (U.S. Pat. No. 3,904,387), a combustible fuel gas is produced by heating solid char or coke with sulfur dioxide. The process produces gaseous carbon monoxide and elemental sulfur. The carbon monoxide is used as a fuel or in petrochemical applications. Moss (U.S. Pat. Nos. 4,041,141 and 4,309,198) converts sulfur dioxide produced from metal sulfides and sulfates to sulfur and carbon oxides, preferably carbon dioxide, by passing the sulfur dioxide through a layer of char. The resulting carbon oxides are vented to the atmosphere. Steiner (U.S. Pat. No. 4,147,762) describes the conversion of sulfur dioxide to sulfur using coal and steam while Daman (U.S. Pat. No. 4,066,738) uses a similar process to recover sulfur from a hydrocarbon fuel. In both cases, the use of steam results in the formation of hydrogen sulfide.
Although various techniques have been developed to reduce the sulfur content of solid and liquid fuels and many of these techniques involve the conversion of sulfur dioxide to elemental sulfur, the full potential of these methods has not been realized in the recovery of chemical values from metal sulfates such as gypsum. In those instances where it has been applied, little if anything has been done to optimize the gaseous products accompanying sulfur production other than the venting of such gases to the atmosphere.